The external ear canal has been used as a basis for dental orientation and measurement since the nineteenth century. Artificial dentures and other dental prostheses are typically fabricated in a laboratory on casts of the jaws mounted on a mechanical articulator which approximates the functional relationships of the jaws. How well the teeth on the articulator imitate natural function depends in a large part on relating them correctly to the joints that control jaw movement.
Positioning the casts of the teeth in the articulator is typically accomplished by a `face bow transfer` which uses an adjustable rigid bow that engages the upper teeth in front and the two ear canals in back. This relies on earposts in the ear canals as the basis for estimating the actual position of the joints, and it is subject to significant error because of the anatomical characteristics of the external ear canal that is engaged by the facebow. Efforts continue today to improve the accuracy and reproducibility of these procedures.
When cephalometric radiography was developed by Broadbent in the 1920's, it was also based on ear canal orientation of the head. The external mechanism is aligned with the x-ray source through movable right and left earposts that engage the outer ear canals. Cephalometric radiographs have become a standard component of orthodontic diagnosis, and the axis connecting the ear canals or related proxy landmarks are still the underlying foundation for measurements derived from these films in all three planes of space.
The orientation of the head on the ear canal axis is controlled by engaging the outer ear canals with a rigidly mounted mechanism usually called a `cephalostat` or `cephalometer,` and this continues to be a major source of error in this technique. Limits of precision exist for every method of measurement, and over time they often become accepted as an integral part of the technique and either factored into measurements or ignored. That is the current state in cephalometrics. The orienting mechanism can be made to any degree of precision, but the anatomy of the engaged ear structures still provides a very poor and inconsistent engagement. Alternative methods proposed up to this time have failed to offer significant improvement.
The external auditory meatus area is irregular in shape, highly variable, mobile, and very sensitive. Those anatomical characteristics make it impossible to engage it with the consistency required for reproducible positioning and precision measurement, but it is all that we have had. The ear canals are the only bilateral structures that can be readily engaged mechanically. Earposts for engaging the canals have been fabricated in various cylindrical, conical or bulbous forms, but all suffer from the same basic problems that prevent predictable or reproducible orientations with the accuracy required for precision applications.